1. Field of the Invention
The present invention relates to a signal detecting method and device, and a radiation image signal detecting method and system that employ correlated double sampling.
2. Description of the Related Art
Photoelectric converters constructed to receive light and output charge signals through the photoelectric conversion, such as CCDs, photomultipliers and the like, are used in various fields. In addition, radiation image recording devices constructed to receive radiation to store charges and output charge signals in proportion to the charges stored therein are also used widely.
As for the detectors for detecting the charge signals outputted from these photoelectric converters and radiation image recording devices described above, integrating amplifiers are generally used, because they can be built into an integrated circuit and have comparatively low noise. The integrating amplifier initiates storage of the charge signals when switched to storage mode, and outputs an electrical signal in proportion to the amount of charges stored therein by discharging the stored charge signals when switched to reset mode.
Here, the switching to the storage mode of the integrating amplifier is implemented by switching the reset switch on the integrating amplifier from ON to OFF. The switching of the reset switch causes kTC noise of the reset switch to be added to the signal component of the electrical signal. In order to avoid the influence of kTC noise, correlated double sampling is implemented, as is described, for example, in the non-patent document “Electronic noise analysis of a 127-micron pixel TFT/photodiode array”, R. L. Weisfield and N. R. Bennett, SPIE, vol. 4320, pp 209-218, 2001. In the method, the difference between an electrical signal outputted from the integrating amplifier when a predetermined baseline sampling time has elapsed after it is switched to storage mode and an electrical signal outputted from the integrating amplifier just before it is switched to reset mode is obtained, and this difference is used as the signal component, thereby the influence of kTC noise is avoided.
In a circuit that performs the correlated double sampling described above, a low-pass filter is used at the latter stage of the integrating amplifier in order to reduce high-frequency noise, and the electrical signal outputted from the integrating amplifier is outputted through the filter.
Low-pass filters, however, have a certain transient response time. Therefore, when a low-pass filter is used as described above, if the baseline sampling time is too short in the correlated double sampling described above, the electrical signal of noise component that has a sufficient magnitude may not be obtained. Consequently, the noise component is added to the signal component, causing degradation of signal-to-noise ratio. Generally, a shorter baseline sampling time is preferable in order to reduce the signal detection time. When a value of the time constant of the low-pass filter is τ, the baseline sampling time is generally set at a value, for example, in the range of 1τ to 2τ, which is not the value determined by taking into account the transient response time.